Core Web Vitals, Explained for People Who Did Not AskCore Web Vitals, Explained for People Who Did Not Ask

Three numbers help decide whether anyone is shown your site at all, and whether the people who do arrive stay. You can check all three in about a minute, without installing anything.

The three

• LCP — how long until the main thing on the page has drawn. Under 2.5 seconds is good.
• CLS — how far the page moved under the reader while it loaded. Under 0.1 is good; this is the one that makes people misclick.
• INP — how long the page took to respond to something they did. Under 200 milliseconds is good.

They are not vanity numbers. They come from real browsers on real connections, they feed into how search ranks you, and each of them corresponds to something a visitor can feel.

What usually breaks them

Almost always the same short list: an oversized hero image, a web font that arrives late and reflows the text, an animation that delays the first paint, and third-party scripts. Tag managers and chat widgets are the most common single cause of a bad score on an otherwise simple site.

<!-- Reserve the space before the image lands, and CLS stays at zero -->
<img src="/hero.avif" width="1200" height="630" alt="" />

That one attribute pair fixes a surprising share of real-world layout shift. The browser can hold the space open only if you tell it how much space to hold.

How to check

Open your site in Chrome, run Lighthouse from the developer tools on a mobile preset, and read the three numbers. Then do it again on a throttled connection, because the desktop-on-fibre reading is the one measurement that never reflects a real visitor.

Every one of these numbers is a description of what your visitor experienced. That is why they are worth reading.